Fig. 3. Flow of the torque vector field.

Streamlines of the N = M × P vector that governs the evolution of the particle orientation P in the damped regime (see main text). Rotational equilibria are the vortex centers of the vector field N. The vortices are indicated with circles and asterisks. Each vortex is also characterized by its topological charge: −1 (asterisks) or +1 (circles). For the current parameters, the poles (θ = 0° and 180°) carry +1 charge and are shown in blue; +1 charges that are also the attractors are colored green. Here, λ = 1064 nm. Notice that the topological invariant Euler characteristic ensures that the sum of all the topological charges equals 2.